1. Field of the Invention
The field of the invention is that of structures integrating a composite material, the material comprising optical fibers designed to check the manufacture and the condition during use of the parts of structures made of composite material, especially multilayer materials. It relates to the composite material included in the structure, a manufacturing process and a method of measurement using the fiber for the localizing and determining of the nature of faults if any.
2. Description of the Prior Art
It is known that a composite material is constituted by fibers assembled in different sheets forming folds or plies of the material.
Depending on the mechanical stresses to which the material will be subjected, the sheets will be arranged in different directions. The composite material is thus formed by different layers or plies. Manufacturing defects may exist between two plies, despite the precautions taken during manufacture. Defects such as these, known as delamination defects, may also appear after manufacture, through ageing, as a result of deformation stresses prompted by vibrations or temperature cycles. These defects may also appear as a result of impacts. Delaminations, irrespectively of their causes, do not prompt any directly visible damage. Now this phenomenon, by creating the possibility of local buckling, considerably reduces the compression strength of the structure. Apart from this mechanical aspect, the heterogeneity thus induced in the material may modify other characteristics. Thus, if the composite material is used for the radome of an antenna, the radiation pattern of this antenna may get modified.
It is therefore necessary, during manufacture and then on a regular basis, to ascertain that the defects in the material do not exceed a threshold that has been set beforehand.
For this purpose, techniques for the non-destructive testing of mechanical structures are used. The techniques generally used are techniques of extrinsic interrogation. This is the case with thermography, acoustic emission, ultrasound and radiography.
All these techniques of checking by extrinsic means have be to applied in laboratories after the structure being checked has been removed. Apart from the disadvantage wherein the equipment has to be removed and remains out of service, the techniques used call for a lengthy learning process in order to be able to have knowledge of the nature of the defects, their dimensions and their localization from the measurement results obtained.
Given the advances made in the manufacture of intrinsic sensors with optical fibers, the use of such sensors has been envisaged through their direct inclusion within the composite material.
An inclusion such as this is reported in an article entitled Ameliorer les performances des composites par fibre optique (Improving the Performance Characteristics of Composite Materials by Optic Fibers) in the journal Electronique International Hebdo, No. 18, 9th May 1991, page 15. This article reports that the project envisaged "consists in the embedding, in a composite material, of a continuous polarization maintaining optical fiber playing the role of an array of sensors, i.e. where the parameter to be measured acts directly on the fiber (unlike extrinsic fibers where the optical fiber is used solely as a medium for the conveying of information). Here, the optical fiber modifies the nature of the signal that travels through it as a function of the deformations and of the temperature. According to the director of the project, with a spacing of 10 cm between each measuring point, the number of points may vary from 20 to 100 depending on the length of the fiber. The primary applications in view are in the aeronautical sector. However [the firm] Bertin, which uses the principle of spectral modulation coding for the processing of measurements, intends to extend the field of application of the array of extrinsic sensors, notably to the measurement of the vibration of a structure".
The main drawback of the measuring method reported here above arises out of the number of steps which is at most equal to the quotient obtained by dividing the width of the total spectrum by the width of the smallest spectrum that it is possible to isolate. According to the method, the meshing of the composite part by the different sensors should be predetermined. It is also necessary to have the ability, through the analysis of the signal picked up at an output of the fiber, to separate the modifications due to the temperature from those due to the mechanical stresses.
The technical problem resolved by the present invention is therefore that of knowing which polarization-maintaining optical fiber is to be used, how to position it inside the composite material, how to replace it in the event of damage and make the material without damaging the optical fiber, and finally, how to make a measurement and use it in order to know the zone in which a defect, if any, might be located.
An aim of the invention, therefore, is to enable the detection and localizing of an irregularity, whether this irregularity is due to a manufacturing defect or to a shock, of a delamination, inside a part made of composite material, the detection and localizing being done by the analysis of the signal going through the fiber without any predetermining of the meshing along the fiber by the nature of the signal that is injected therein.
The aim is also to detect stresses or modifications of stresses located within a predetermined range. Finally, these detections and localizations of irregularities can be made in real time or in deferred time. Real-time analysis is especially worthwhile when the composite part is the radome of an antenna for which it is important to ensure that the radiation pattern remains constant, for example an antenna radome for an instrument landing system (ILS). In every case, the invention enables the detection of defects by means of a single instrument, for example a Michelson interferometer with movable mirror, without any need to remove the part.